UTILITY PLANT AND JOINTLY-OWNED FACILITES (Utility Plant in Service by Major Class) (Details) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2016	Dec. 31, 2015	Dec. 31, 2014
Public Utility, Property, Plant and Equipment [Line Items]
Depreciation Rate	2.85%	2.83%	2.99%
Total Plant in Service	$ 5,975,139	$ 5,618,435
Utility Plant Under Capital Leases	167,413	131,705
Plant Acquisition Adjustments	$ (139,000)	(97,000)
Generation Plant
Public Utility, Property, Plant and Equipment [Line Items]
Depreciation Rate	3.31%
Average Remaining Life of Public Utilities Property Plant and Equipment	22 years
Total Plant in Service	$ 2,866,000	2,612,000
Transmission Plant
Public Utility, Property, Plant and Equipment [Line Items]
Depreciation Rate	1.48%
Average Remaining Life of Public Utilities Property Plant and Equipment	32 years
Total Plant in Service	$ 1,024,000	1,008,000
Distribution Plant
Public Utility, Property, Plant and Equipment [Line Items]
Depreciation Rate	2.08%
Average Remaining Life of Public Utilities Property Plant and Equipment	35 years
Total Plant in Service	$ 1,512,000	1,456,000
General Plant
Public Utility, Property, Plant and Equipment [Line Items]
Depreciation Rate	5.48%
Average Remaining Life of Public Utilities Property Plant and Equipment	11 years
Total Plant in Service	$ 381,000	358,000
Intangible Plant, Software Costs and Other (1)
Public Utility, Property, Plant and Equipment [Line Items]
Total Plant in Service	185,000	179,000
Capitalized Computer Software, Net	52,000	45,000
Amortization of computer software costs	17,000	14,000	$ 17,000
Plant Held for Future Use
Public Utility, Property, Plant and Equipment [Line Items]
Total Plant in Service	$ 7,000	$ 5,000
Enterprise Software [Member]
Public Utility, Property, Plant and Equipment [Line Items]
Average Remaining Life of Public Utilities Property Plant and Equipment	3 years
Minimum [Member] | Application Software [Member]
Public Utility, Property, Plant and Equipment [Line Items]
Acquired Finite-lived Intangible Assets, Weighted Average Useful Life	3 years
Maximum [Member] | Application Software [Member]
Public Utility, Property, Plant and Equipment [Line Items]
Acquired Finite-lived Intangible Assets, Weighted Average Useful Life	5 years